1. Field of the Invention
The present invention relates to a rack and pinion type power steering apparatus characterized by a rack bush sidably supporting a rack shaft and an attaching structure of the bush.
2. Description of the Related Art
In a rack and pinion type steering apparatus, a structure is well known in which a rack shaft is slidably supported to a rack housing corresponding to a support body of the rack shaft at two positions. These structures comprise a gear housing position corresponding to a position close to one end of the rack shaft corresponding to an engagement portion between a rack gear of the rack shaft and a pinion, and the other housing position corresponding to a position close to the other end. The rack shaft is supported to the rack housing at these positions via a rack bush made of synthetic resin, rubber or the like (refer, for example, to Japanese Unexamined Patent Publication No. 2000-177606 (patent document 1) (page 4, FIG. 1)).
The slidable support of the rack shaft 01 to the rack housing 02 described in the patent document 1 mentioned above is accomplished at two positions comprising one end portion 05 corresponding to a position at which a rack gear 03 in both ends portions of the rack shaft 01 and a pinion 04 are engaged with each other, and the other end portion 06 of the rack shaft 01, as illustrated in FIG. 6. Each of first and second rack bushes 07 and 08 used in the support of the one end portion 05 and the other end portion 06 of the rack shaft 01 is made of a synthetic resin or the like. These rack bushes 07 and 08 are provided with three support portions which support the rack shaft 01 in a contact manner, slightly protruding from the inner peripheral surfaces thereof and are not clearly illustrated, in the inner peripheral surface. These three support portions in the first bush 07 and the second bush 08 are arranged in the inner peripheral surfaces of the respective bushes 07 and 08 in such a manner that a phase in a peripheral direction of the peripheral surface of the rack shaft 01 is shifted with each other.
In this case, the support of the rack shaft 01 by the rack bushes 07 and 08 in two positions comprising the one end portion 05 and the other end portion 06 of the rack shaft 01 mentioned above is executed by using the bushes 07 and 08 provided with a specific support structure. The support position of the rack shaft 01 includes the engagement portion between the rack gear 03 and the pinion 04 corresponding to the support portion of the one end portion 05 of the rack shaft 01, and the other end portion 06 which is relatively distant from the engagement portion. The support of the rack shaft 01 at two relatively distant positions in both ends of the rack shaft 01 generates an oscillation of the rack shaft 01 in spite of the use of the bushes 07 and 08 provided with the specific support structure mentioned above. The generation of the oscillation in the rack shaft 01 makes it difficult to secure a proper engagement between the rack gear 03 and the pinion 04, and it is very difficult to inhibit a gear engaging sound from being generated due to the engagement of the gears.
Under these circumstances, it is desirable to improve the rack and pinion type power steering apparatus mentioned above. The improvement involves a selective setting of a proper support position to the housing by the rack bush of the rack shaft and the support structure of the rack bush and the rack shaft by the bush, in the slidable support to the housing by the rack bush of the rack shaft. It is thereby possible to effectively inhibit an oscillation of the rack shaft from being generated, to intend to make the engagement between the rack gear of the rack shaft and the pinion proper, and to effectively inhibit the gear engaging sound generated when the gears are engaged.